[SRU][R][PATCH 0/1] dma-buf filesystem flags fix

[AceLan Kao]

From: "Chia-Lin Kao (AceLan)" <acelan.kao at canonical.com>

BugLink: https://bugs.launchpad.net/bugs/2139656

[Impact]
The VFS layer now enforces that inodes flagged with S_ANON_INODE must be located on
filesystems with the SB_I_NOEXEC flag set. The dmabuf (Direct Memory Access Buffer)
filesystem creates inodes using alloc_anon_inode(), which sets the S_ANON_INODE flag
but does not set SB_I_NOEXEC and SB_I_NODEV on the filesystem.

This causes a kernel warning when a dmabuf is memory-mapped, which can occur in
multimedia applications such as GStreamer's v4l2src (Video4Linux source) element.
The warning does not cause a crash but indicates a VFS policy violation that will
eventually become a fatal error in future kernel versions.

Error log:
```
[ 60.061328] WARNING: CPU: 2 PID: 2803 at fs/exec.c:125 path_noexec+0xa0/0xd0
...
[ 60.061637] do_mmap+0x2b5/0x680
```

Affected scenarios: Any system running multimedia applications that use video capture
or memory-mapped dmabuf operations, particularly with Video4Linux source elements or
similar hardware-accelerated media processing.

[Fix]
Set the SB_I_NOEXEC and SB_I_NODEV flags on the dmabuf filesystem context. This follows
the same pattern previously applied to other anonymous inode filesystems such as
secretmem (commit 98f99394a104c) and the anon_inode filesystem (commit ce7419b6cf23d).

The fix is minimal - adding two flag assignments in drivers/dma-buf/dma-buf.c:
- fc->s_iflags |= SB_I_NOEXEC;
- fc->s_iflags |= SB_I_NODEV;

These changes align the dmabuf filesystem with the VFS enforcement introduced in
commit 1e7ab6f67824 ("anon_inode: rework assertions").

Upstream patch: https://lore.kernel.org/lkml/20260114011917.241196-1-acelan.kao@canonical.com/

[Test Plan]
1. Install a system with a video capture device (USB webcam, integrated camera, or
   Video4Linux-compatible device)

2. Install GStreamer and the v4l2src plugin:
   $ sudo apt-get install gstreamer1.0-plugins-good gstreamer1.0-plugins-base

3. Run a GStreamer pipeline that uses v4l2src to capture video:
   $ gst-launch-1.0 v4l2src ! video/x-raw, width=640, height=480, framerate=30/1 ! videoconvert ! autovideosink

4. Check the kernel log for warnings:
   $ dmesg | grep -i "path_noexec\|SB_I_NOEXEC"

Expected result without fix:
- WARNING message appears in dmesg at fs/exec.c:125 (path_noexec+0xa0/0xd0)
  indicating anonymous inode on non-compliant filesystem

Expected result with fix:
- No warning appears; video capture works correctly without VFS policy violations

[Where problems could occur]
The dmabuf filesystem itself could be affected if the SB_I_NOEXEC or SB_I_NODEV flags
cause unexpected behavior with the filesystem's inode operations.

If there are edge cases where the dmabuf filesystem needs to allow execution or device
access for specific operations, this fix could prevent those operations from working.
Symptoms would include dmabuf allocations failing, memory mapping failing, or hardware
acceleration features becoming unavailable.

The SB_I_NODEV flag prevents device node creation on the filesystem (which should never
occur on a pseudo-filesystem), and SB_I_NOEXEC prevents execution (which is the correct
behavior for a memory buffer filesystem). These are defensive flags used by all
similar anonymous inode filesystems, so the regression risk is minimal, but if there
are hidden dependencies on the absence of these flags, dmabuf clients could fail.

Monitor for:
- Memory mapping failures in applications using dmabuf
- Failures to create dmabuf file descriptors
- DMA operations failing to map memory regions
- Video capture or media acceleration features becoming unavailable

[Other Info]
This is a backport of a fix for the upstream kernel. The patch follows the exact
approach used in previous commits to other anonymous inode filesystems:
- commit 98f99394a104c ("secretmem: use SB_I_NOEXEC")
- commit ce7419b6cf23d ("anon_inode: raise SB_I_NODEV and SB_I_NOEXEC")

The warning was introduced by commit 1e7ab6f67824 ("anon_inode: rework assertions")
which added enforcement in the VFS layer that anonymous inodes must have these flags
set on their host filesystem.

Chia-Lin Kao (AceLan) (1):
  UBUNTU: SAUCE: dma-buf: set SB_I_NOEXEC and SB_I_NODEV on dmabuf
    filesystem

 drivers/dma-buf/dma-buf.c | 2 ++
 1 file changed, 2 insertions(+)

-- 
2.53.0